Prior-art methods of this type are based on the so-called extruder weld principle, according to which a strand of molten plastic material from an extruder nozzle is gradually fed into a groove of preferably V-section, which is formed between the tubular plastic products to be joined together. During the welding the extruder nozzle must be passed along the groove substantially at the same speed as the strand of plastic mix is pressed out of the extruder nozzle. In order that a tight and reliable joint could be obtained, complete synchronization must prevail between the feeding velocity of the plastic mix and the extruder nozzle. The extruder may be either shaped as a hand tool, in which case high professional skill is required, or it may be mounted on a welding machine, wherein the feeding of the plastic mix and the drive of the tubular plastic product or of the extruder nozzle take place completely automatically. It is a drawback of extruder welding that it is very slow and requires relatively complicated equipment if the work is to be carried out mechanized.
In particular in the case of installation of insulated offshore pipelines which comprise an inner metal pipe for transfer of medium and a coextruded protective and insulating layer of plastic surrounding the metal pipe, it is required that the welding of the protective and insulating layer must not take a longer time than the welding together of two metal pipes. The protective and insulating layer comprises an inner foamed plastic material, such as foamed polyolefin or equivalent, and an outer protective mantle of a smooth and liquid-tight plastic, likewise polyolefin or some other weldable plastic. The pipeline is prefabricated in certain lengths with just a little portion at each end free from the surrounding protective and insulating layer to permit welding together of the metal pipes. On completion of the welding, the platform on which the work is performed is driven forwards along the length of the pipeline piece, whereby the newly welded joint arrives at the station at which the joint is to be provided with a protective and insulating layer similar to that surrounding the rest of the pipeline. For this purpose it is possible to utilize prefabricated cylindrical, axially split-open joint pieces with chamfered edges, which joint pieces must be fixed by welding with absolutely liquid-tight seams, two peripheral circular seams along the ends of the joint piece and one axial straight seam along the split slot of the joint piece.
With extruder welding there is no chance to attain a welding speed that is required to keep pace with the metal pipe welding.